Various metal alloys possess the ability to change their shape as a result of a change in temperature. Such SMA can undergo a reversible transformation from a martensitic state, in which the material is relatively soft and deformable, to an austenitic state in which the material possesses super elastic properties and is relatively firm. The transformation from the martensitic state to the austenitic state will be referred to herein as the "austenitic transformation", and the other transformation, from the austenitic state to the martensitic state, will be referred to herein as the "martensitic transformation". The austenitic transformation occurs over a range of temperature which is higher than the range of temperatures in which the reverse transformation occurs. This means, that once transformed to an austenitic state, an SMA will remain in that state even when cooled to a temperature below that in which the austenitic transformation began, as long as the temperature is above that in which the martensitic transformation begins.
A particular class of SMAs are alloys of nickel and titanium--Nitinol. Nitinol has found a variety of uses in medical as well as other fields. Medical uses of SMAs, particularly Nitinol, has been described in U.S. Pat. Nos. 4,665,906, 5,067,957, European Patent Application 143,580, U.S. Pat. No. 4,820,298 and many others.
For medical uses it is usually desired that the alloy will undergo an austenitic transformation over a narrow, well defined range. For example, a vascular stent of the two-way SMA type, such as that described in European Patent Application, Publication No. 625153, is typically deployed in the body while being in the martensitic state at body temperature, and then after heating, it transforms into the austenitic state, and then remains in the austenitic state when cooled to the body temperature. It can be appreciated that if excessive heating to transform the SMA from the martensitic to the austenitic state is required, this can be damaging to the surrounding tissue and is thus undesirable. Thus, it would ideally be desired that the austenitic transformation will begin at a temperature several degrees above body temperature and will be over a temperature range which will not cause tissue damage owing to the excessive heating.